1. Field of the Invention
The present invention relates in general to data transmission systems, and in particular to data transmission systems which may be utilized in wellbores to communicate coded messages through fluid columns disposed therein.
2. Description of the Prior Art
In the oil and gas industry, it has been one longstanding objective to develop data transmission systems which do not require the utilization of electrical conductors to carry coded signals between wellbore locations which are separated by great distances. Experience has revealed that data transmission systems which require the utilization of electrical conductors extending between communication nodes in a wellbore are advantageous when data must be communicated within the wellbore at extremely fast transmission rates, or when large blocks of data need to be transferred between communication nodes; however, the utilization of electrical conductors has several serious disadvantages including: (1) since most wellbores include regions which are exposed to corrosive fluids and high temperatures, a long service life cannot be expected from a data transmission system which utilizes electrical conductors; (2) since most wellbores extend for substantial distances, data transmission systems which utilize electrical conductors are not generally considered to be cost effective, particularly when such systems are utilized only infrequently, or in a limited manner; (3) since all wellbores define fairly tight operating clearances, utilization of a wireline conductor to transmit data may reduce or diminish the operating clearance through which other wellbore operations are performed; and (4) since wellbores typically utilize a plurality of threaded tubular members to make up tubular strings, utilization of an electrical conductor to transmit data within the wellbore complicates the make-up and break-up of the tubular string during conventional operations.
Accordingly, the oil and gas industry has moved away from the utilization of electrical conductor data transmission systems (frequently referred to as "hardwire" systems), and toward the utilization of pressure changes in a fluid column to transmit data within the wellbore. One example of the extensive use of fluid columns within a wellbore to transmit data is that of measurement-while-drilling data transmission systems, also referred to as "MWD" systems. Typically, these systems are utilized only in drilling operations. Generally, a plurality of sensors are provided in a tubular subassembly located within the bottom hole assembly, near the rock bit which is utilized to disintegrate the formation. The electrical sensors detect particular wellbore parameters, such as temperature, pressure, and vibration, and develop electrical signals corresponding thereto. The electrical signals are converted into a digital signal stream (generally multiplexed sensor data) and utilized to develop a plurality of pressure changes in a fluid column, typically the tubing fluid column, which are sensed at the earth's surface and converted into a format which allows the drilling engineers to make decisions which affect the drilling operations. Some attempts have been made to apply the concepts of MWD data transmission systems to completion operations, during which the drilled wellbore is placed in condition for continuous production of oil and gas from selected wellbore regions. To date, no entirely satisfactory data transmission system has been developed for utilization in completion operations, nor has a satisfactory data transmission system been developed for utilization during prolonged production periods.
At present, several standard approaches are utilized to operate wellbore tools which are remotely located within a wellbore on a workstring. One approach is to increase the pressure of fluid disposed within the central bore of the workstring until a predetermined pressure threshold is exceeded, causing the shearing of set screws or other similar pressure-sensitive mechanical latches. Another approach is to utilize a slickline to pass an actuating tool, with a particular profile, into contact with a profiled component on the wellbore tool. Yet another approach is to utilize an electrical-conductor (commonly referred to as an "E-line") to energize an electrically-actuated substance which operates mechanical components to switch a wellbore tool between modes of operation.